1. Field of the Invention
The present invention relates to a method for wireless data transmission between a base station and one or more transponders and also a modulation control device for carrying out the method.
2. Description of the Background Art
Such transmission methods between one or more base stations or readers and one or more transponders are used, for example, in contactless identification systems or so-called radio frequency identification (RFID) systems. Sensors, for example, for temperature measurement, may also be integrated in the transponders. Such transponders are also referred to as remote sensors.
The transponders and their transmitting and receiving devices customarily do not have an active transmitter for data transmission to the base station. Such non-active systems are called passive systems if they do not have their own energy supply, and semi-passive systems if they have their own energy supply. Passive transponders take the energy they require for their supply from the electromagnetic field emitted by the base station.
In general, so-called backscatter coupling is used to transmit data from a transponder to the base station using UHF or microwaves in the far field of the base station. To this end, the base station emits electromagnetic carrier waves, which the transmitting and receiving device in the transponder modulates and reflects appropriately for data to be transmitted to the base station using a modulation method. The typical modulation methods for this purpose are amplitude modulation, phase modulation and amplitude shift keying (ASK) subcarrier modulation of the backscatter signal, in which the frequency and/or the phase position of the subcarrier is changed.
The data transmission from a transponder to the base station can occur synchronously with synchronization markers or so-called “notches” that are transmitted by the base station.
A variety of methods are known for producing the synchronization markers. In general, the carrier signal is amplitude-modulated and/or phase-modulated with a modulating signal by the base station in such a manner that a detection unit within the transponder, which is basically designed for amplitude modulation of the carrier signal, can detect the synchronization marker. A synchronization marker is typically detected in a transponder using what is known as an RSSI circuit.
Methods for increasing the transmission range are known which do not completely blank the carrier signal during the modulation period, i.e., which have a modulation index that is less than one.
To reduce the bandwidth required, the modulation signal can have a sine or cosine shape during the modulation period, i.e. the carrier signal is not blanked with a square-wave function, but instead is switched off and then back on again with a sinusoidal profile. An example of this is the method known as double sideband modulation (DSBM) with a suppressed carrier. When a cosine function is used as the modulation signal, the DSBM can be described mathematically by the following basic equation:
                    u        m            ⁡              (        t        )              =                  C        *                  cos          ⁡                      (                                          ω                0                            *              t                        )                          *                  cos          ⁡                      (                          Ω              *              t                        )                              ⁢                          ⁢                          =                                    C            2                    *                      cos            ⁡                          (                                                (                                      Ω                    +                                          ω                      0                                                        )                                *                t                            )                                      +                              C            2                    *                      cos            ⁡                          (                                                (                                      Ω                    -                                          ω                      0                                                        )                                *                t                            )                                            ,where um(t) represents the modulated signal, C represents a constant, ω0 represents the angular frequency of the modulation signal, Ω represents the angular frequency of the carrier signal, and t represents the time.
The carrier frequency here is suppressed in the spectrum of the modulated signal um(t). This type of generation of synchronization markers improves utilization of the available bandwidth as compared to modulation methods with no carrier suppression. Moreover, the supply of energy to the transponder can be improved.
The data to be transmitted consist of a sequence of symbols, each of which customarily are coded and transmitted by a transponder within a predefined time interval or symbol interval. The time interval or symbol interval is customarily determined by two successive synchronization markers. In a binary transmission, the value of a symbol is either “0” or “1.”
Various methods for coding a symbol are known. A first coding method is known as non-return to zero (NRZ) coding. The modulation state of a serially transmitted signal, known as a line code, corresponds here to the value of the symbol to be transmitted. A variation of this method is known as non-return to zero inverted (NRZI) coding. In this method, a “1” is represented by a change in the modulation state of the serially transmitted signal at the beginning of a symbol interval or bit interval. When a “0” is transmitted, no change in the serial signal takes place. The aforementioned methods are described, for example, in the textbook by Klaus Finkenzeller, RFID-Handbuch, 3rd edition, HANSER, 2002, see especially Chapter 6.1, Codierung im Basisband (coding in the baseband).
If the coding or a change in the modulation state in the transponder takes place approximately simultaneously with the synchronization markers transmitted by the base station, there exists the danger that the generation or transmission of the synchronization markers will make it more difficult to evaluate the simultaneously backscattered signals, which have a very low power.
If, for example, a synchronization marker is generated with the aid of double sideband modulation, it is customarily done with the aid of an IQ modulator whose output signal is amplified by a power amplifier. The modulation signal is applied to the I connection of the IQ modulator.
In this regard, it is frequently necessary in conventional systems to monitor and, if necessary, adjust the zero-point symmetry of the resulting envelope curve in order to transmit a constant power, among other reasons. When such an asymmetry arises, this leads to a change in the magnitude of the signal backscattered by a transponder. Additional nonlinearities can also have an effect on the backscattered signal. Such effects on the backscattered signal, traceable to the generation of the synchronization markers, significantly complicate evaluation of the signal. Consequently, it is frequently useful overall to monitor and, if necessary, adjust the modulated carrier signal. Alternatively, adaptive systems are also used, which evaluate the present backscattered signals on the basis of past transmissions or data. The methods mentioned have in common that they, however, require significant implementation complexity in the base station.